Geant4 simulation study of β-γ coincidence detector for 41Ar measurement

[Background] As one of the main radionuclides released into the environment by the operation of reactors and accelerators, the measurement of 41Ar activity concentration is of great significance for ensuring public health. In the field of radioactive gas measurement, the β-γ coincidence method is widely used because it can significantly reduce the background and improve the sensitivity of the detector. However, present, there is little study on β-γ coincidence detectors for 41Ar measurement. [Purpose]This study aims to optimize the β-γ coincidence detector composed of plastic scintillator and CsI(Tl) scintillator to realize high sensitivity measurement of 41Ar. [Methods] An optimization method of detector structure based on minimum detectable activity concentration (MDC) is proposed. The optimization process of the detector is realized based on Geant4 simulation. Firstly, the penetration percentage of β-rays depositing energy into CsI(Tl) through different thicknesses of BC404 and 0.5 mm aluminum film was simulated. Secondly, the full-energy peak efficiency of 1 293.6 keV γ-ray in CsI(Tl) scintillator with different thicknesses was simulated. Thirdly, assuming that the sampling time of argon is proportional to the volume of the detector’s gas chamber, by simulating with different gas chamber volume, the comprehensive effects of β-rays detection efficiency, γ-ray full-energy peak efficiency, gas chamber volume and sampling time on MDC were analyzed. Finally, the influence of measurement time on MDC under different background counting rates was analyzed. [Results] It is concluded that the penetration percentage of β-rays emitted from the decay of 41Ar is about 0.78% when the thickness of BC404 is 3 mm. The γ-ray full-energy peak efficiency increases with the increase of CsI(Tl) scintillator thickness. With the argon sampling rate of 600 mL∙h−1, the optimal detector size parameters that minimize MDC are completely determined. When the background count rate is 1×10-3~1 s-1, the recommended measurement time for 41Ar is about 200 min. [Conclusions] When the measurement time is 200 min, the sample cooling time is 30 min and the background count rate is 5×10-3s-1, the MDC of the optimized β-γ coincidence detector for 41Ar measurement is about 1.7 Bq∙m−3.

[背景] 作为反应堆与加速器运行过程中向环境释放的主要放射性核素之一，氩-41（⁴¹Ar）的活度浓度测量对于保障公众健康具有重要意义。在放射性气体测量领域，β-γ符合方法（β-γ coincidence method）因可显著降低本底、提升探测器灵敏度而得到广泛应用。但目前针对氩-41测量所用的β-γ符合探测器的研究相对较少。
[目的] 本研究旨在优化由塑料闪烁体（plastic scintillator）与碘化铯(Tl)闪烁体（CsI(Tl) scintillator）组成的β-γ符合探测器，以实现氩-41的高灵敏度测量。
[方法] 提出了一种基于最小可探测活度浓度（minimum detectable activity concentration, MDC）的探测器结构优化方法，并基于Geant4模拟实现探测器的优化流程。首先，模拟了β粒子穿过不同厚度BC404塑料闪烁体与0.5mm铝膜后，在碘化铯(Tl)闪烁体中沉积能量的穿透率；其次，模拟了不同厚度的碘化铯(Tl)闪烁体对1293.6 keV γ射线的全能峰效率；随后，假设氩气采样时长与探测器气室体积成正比，通过改变气室体积进行模拟，分析了β粒子探测效率、γ射线全能峰效率、气室体积与采样时长对最小可探测活度浓度的综合影响；最后，分析了不同本底计数率下测量时长对最小可探测活度浓度的影响。
[结果] 研究得出，当BC404厚度为3mm时，氩-41衰变产生的β粒子穿透率约为0.78%；γ射线全能峰效率随碘化铯(Tl)闪烁体厚度的增加而提升；在氩气采样速率为600 mL·h⁻¹的条件下，确定了使最小可探测活度浓度达到最优的探测器尺寸参数；当本底计数率处于1×10⁻³~1 s⁻¹区间时，氩-41的推荐测量时长约为200 min。
[结论] 当测量时长为200 min、样品冷却时长为30 min、本底计数率为5×10⁻³ s⁻¹时，经优化的氩-41测量用β-γ符合探测器的最小可探测活度浓度约为1.7 Bq·m⁻³。